99 Chestnut St and 76 Chestnut Street are on different sides of the Street and part of different associations. While mice and rats do not recognize property boundaries, and can be found on both sides of the street in one general area, different associations may be more proactive in their pest prevention policies. The best place to get comprehensive information would be at the Brookline Health Department. Go to brooklinema.gov and go to the health department section to find hours to call someone directly.
Good Luck in your home search.